The present invention relates to an electromagnetically operated proportional valve, in particular a pressure regulating valve for automatic transmission of motor vehicles.
Such proportional valves are known in the art. One of such valves is disclosed for example in the German document DE 88 09 143 U1. The proportional valves of this type have to satisfy in operation the requirements with regard to service life and quality. In particular, a constant valve characteristic line (pressure/exciting current characteristic line or throughflow-exciting current characteristic line) must be guaranteed over a maximum long service life. Such proportional valve however can operate in more or less contaminated pressure medium circulation, and therefore due to leakages from pressure chamber and armature chamber ferromagnetic contaminating particles can accumulate in the armature chamber. In particular, when the armature chamber of the proportional magnet for producing a hydraulic damping is filled with a damping fluid (for example pressure medium), the inflow of the contaminating particles is facilitated. These ferromagnetic contaminating particles are deposited in the armature chamber and accumulated at the locations with greater magnetic field density. The area with the greater magnetic field density is as a rule, the working air gap of the magnetic circuit. Thereby in operation during the service life of the proportional valve a displacement of the valve characteristic line due to the influence of the magnetic resistance in the working air gap occurs. With growing deposits of ferromagnetic particles in the working air gap a bridge formation takes place and can lead to failure of the valve.